When disorders in blood vessels occur in vivo, platelets and/or coagulation cascades are activated for preventing blood leakage to form thrombi, which in turn suppress hemorrhage. Thrombin formed by the coagulation cascade activation cleaves fibrinogen to form insoluble fibrin. Fibrin is present in the form of a network in thrombi and works to strengthen the thrombi. This reaction is called coagulation. The formed fibrin is then degraded through in-vivo reaction. This reaction is fibrinolysis. Under normal conditions, coagulation and fibrinolysis are balanced, and abnormal amounts of thrombi do not accumulate in blood vessels. However, once the balance is disrupted to accelerate coagulation, it may come into a state that a thrombus is likely to be formed in blood vessels, leading to various diseases attributed to thrombosis. The thrombus formation is caused by three factors (Virchow's triad: change in the properties of vascular walls, change in blood components, and change in blood flow). Diseases attributed to the thrombus formation are one of the most general causes of death among advanced nations.
TAFI (thrombin-activatable fibrinolysis inhibitor) is a carboxypeptidase that is produced in the liver and secreted into blood. This enzyme is activated through the cleavage of N-terminal 92 amino acid residues by thrombin or thrombin/thrombomodulin complexes. TAFI is also called procarboxypeptidase U, procarboxypeptidase R, or plasma procarboxypeptidase B.
The activated TAFI is called TAFIa. TAFIa inhibits fibrinolysis by removing the C-terminal Lys or Arg residue of fibrin or fibrin degradation products (FDPs), which are main components of thrombi. Two enzymes, tPA (tissue-type plasminogen activator) and plasminogen, which induce and promote fibrinolysis, bind to the Lys residue of fibrin or FDPs via their Lys-binding sites. On the surface of the fibrin molecule, tPA subsequently activates plasminogen and converts it into plasmin to initiate fibrinolysis. Plasmin cleaves fibrin, and a Lys or Arg residue appears at the C-termini of the formed FDPs. The continuation of fibrinolysis allows plasminogen and tPA to newly bind to the Lys residues of the FDPs to further form plasmin. This efficiently promotes fibrinolysis (positive feedback mechanism of fibrinolysis). TAFIa inhibits the plasminogen activation of tPA on the fibrin molecule by removing the C-terminal Lys residues of FDPs. As a result, efficient fibrinolysis does not occur. TAFIa suppresses the positive feedback mechanism of fibrinolysis. These findings are described in detail in a review on TAFI and its inhibitors (Non Patent Literature 1).
As described above, the fine balance between coagulation and fibrinolysis is achieved in vivo. When coagulation is accelerated by diseases or the like, thrombi come to be likely to be formed, developing various diseases. Such diseases include myocardial infarction, angina pectoris, acute coronary syndrome, cerebral infarction, deep vein thrombosis, pulmonary embolism, peripheral arterial occlusion, sepsis, disseminated intravascular coagulation syndrome, and pulmonary fibrosis.
The previous treatment of thrombosis has often targeted enzymes in the coagulation cascades. These enzymes include activated coagulation factor X (Xa), thrombin, and the like. Inhibitors against these enzymes have the risk of potential adverse reaction such as hemorrhage. Heparin or low-molecular-weight heparin cannot be expected to exert drug efficacy in oral administration and requires administration in hospitals. Warfarin is orally administrable but requires periodic blood tests by reason of interaction with other drugs, etc. Aspirin is an orally administrable drug that inhibits thrombus formation by suppressing the activation of platelets, but has adverse reaction such as gastrorrhagia. A goal for further improving the current therapies is to prevent bleeding time from being prolonged while maintaining high therapeutic effect by drug administration. TAFIa inhibitors are thought to have a small risk of hemorrhage, because they do not influence the process of hemostasis involving coagulation and platelets.
In pathologies where it may arise that a thrombus is likely to be formed due to accelerated coagulation reactions, thrombi can be removed more quickly by making fibrinolysis efficient through the inhibition of TAFIa. This can be expected to exert excellent effects on the treatment/prevention of diseases attributed to thrombi. Some cases of animal experiments that showed an antithrombotic effect by inhibiting TAFIa have been reported so far.
There is a report that the intravenous administration of a TAFIa-inhibiting polypeptide consisting of 39 amino acids (potato carboxypeptidase inhibitor (PCI)) to mice showed an antithrombotic effect in iron chloride-induced thrombus models (Non Patent Literature 2).
A low-molecular-weight TAFIa inhibitor reduced the amount of thrombi by approximately 35% in intravenous administration to rabbit models of venous thrombosis (Non Patent Literature 3).
A low-molecular-weight TAFIa-inhibiting compound showed, in rat models of thromboembolism, a reduction in the amount of thrombus deposits in the kidney with the effect of increasing a fibrinolysis marker D-dimer as well as comparable antithrombotic effect at a reduced dose of tPA in combined use with tPA (Non Patent Literatures 4 and 5).
Patent Literatures 1 to 5 disclose compounds that exhibit TAFIa inhibitory activity.